Electrochemical double layer capacitor or super/ultra-capacitors have been known for many years but have been used sparsely in power applications. A standard electrochemical double layer capacitor or super/ultra-capacitor traditionally consists of two high surface area electrodes that are polarized with the use of an electrolytic acid. The capacitance (C) of plate capacitors can be calculated with equation C=(ε*A)/d, where ε is the permittivity of the capacitor material, A is the surface area of the electrode plate, and d is the distance between plates. The charges in a super/ultra-capacitor are held between two ionic charges (distance d) called the Helmholtz layer. Due to the very short distance d, typically less than 0.4 μm, a very high capacitance compared to conventional capacitors can be achieved using the Helmholtz layer.
The characteristics of super/ultra-capacitors include very fast charge time and high instantaneous power bursts. Since a super/ultra-capacitor does not rely on an electrochemical reaction, it also does not suffer from a “memory effect” that is often seen in batteries. Therefore, super/ultra-capacitors can bridge the gap between conventional capacitor (high power/low energy) and electrochemical devices such as rechargeable batteries (low power/high energy). In addition, super/ultra-capacitors are often seen as a “green” alternative compared to traditional batteries. The lifetime of super/ultra-capacitors can run maintenance free for 10-15 years; thus replacing traditional batteries with super/ultra-capacitors can reduce hazardous chemical wastes.
However, mass adoption has been slow for electrochemical double layer capacitors or super/ultra-capacitors due to their low energy storage capacity compared to rechargeable batteries and relatively high cost for manufacturing. Applications of super/ultra-capacitors are limited mostly to industrial applications, such as regenerative braking for large trains or engine start for large commercial diesel trucks. There remains a need for electrochemical double layer capacitors or super/ultra-capacitors with higher energy storage capacity and low cost manufacturing processes in order to meet the increasing demand for fast charging and high capacity energy storage devices in today's emerging electronic applications like mobile phones and electric vehicles.